1. Field of the Invention
The invention is directed generally to electrical extension cords and power supply cords and, more specifically, to electrical extension and power supply cords having built in safety protection and diagnostics.
2. Description of the Prior Art
The electrical extension cord in use today includes a plug, usually comprising two or three prongs, an electrical conducting cord typically comprising two or three insulated wires several feet in length and a terminal connector or receptacle for receiving one or more electrical plugs to power lamps, a television, household appliances, an air conditioner, etc. A grounded extension cord includes a plug having three prongs and a three conductor insulated wire cord where two conductors are utilized for phase and neutral or return power and the third conductor is used as a common ground. While extension cords provide many advantages, there are some disadvantages that are also associated with their use. For example, extension cords are often left underneath rugs where they are trampled upon, or they are pinched by doors and furniture which can lead to arcing or short circuiting which can cause a fire. Extension cords also frequently tend to be left coiled where heat can concentrate, or are overloaded to the point of destruction by fire. Given the number of dangerous situations which can develop pursuant to extension cord use and abuse, such as residential fires and electrical shock, an extension cord design which offers some protection in anticipation of homeowner/user abuse is desired.
U.S. Pat. No. 5,642,248 assigned to Leviton Manufacturing Co., Inc. discloses an electrical extension cord where the insulated phase, neutral and ground conductors are surrounded by a braided sensing shield. The braided shield is electrically connected at the receptacle to the ground conductor and extends to the plug. Leakage current released from the conductors may be collected in the shield and detected by a Ground Fault Circuit Interrupter (GFCI). The purpose of the shield is to capture any type of leakage current within the extension cord and transfer it to ground such that the GFCI may detect the current imbalance and interrupt the circuit. This type of device is commonly known as a Leakage Current Detector Interrupter (LCDI).
Present day GFCI based leakage current detectors have several limitations. One such limitation is that of being a relatively expensive and complex device which requires the use of one or more toroidal transformers to function. These transformers can be very large for high current applications. In addition, presently available devices require that a ground be available at the outlet that the leakage current detector is plugged into. This may not always be the case in residential circuits, and some applications, such as hospitals, require a floating ground.
Two other problems are inherent in circuit interrupting devices of various manufacturers presently available that can be plugged into household outlets. One such problem is that it may be possible to reset a circuit interrupting device to provide power when the fault detecting and interrupting circuitry is no longer functioning. It is here noted that this problem has been addressed and solved in GFCIs manufactured by Leviton Manufacturing Co., Inc. of Little Neck, N.Y. With the Leviton device, known in the industry as a reset lockout GFCI, the main contacts cannot be closed when the device fails to operate the interrupting mechanism.
Another problem that can occur is when a device is plugged into an outlet in such a way that electricity is supplied to the phase terminal but not to the neutral terminal. When this happens the interrupting device is not powered and therefore can not operate because there is no return or neutral circuit. But, high voltage is still available to the user and, therefore, a potentially dangerous situation can exist. Relays that are non-latchable and normally open contact are typically used to prevent high voltage being available to the user when this type of open neutral condition exists. The normally open relay will not close the contacts unless a return circuit is available, and will open if the neutral circuit should open. However, this type of relay is relatively large and consumes a large amount of power.
It is now a requirement that window air conditioners be protected with a Leak Current Detector Interrupter (LCDI). Therefore, what is needed is an extension cord that contains diagnostics which can indicate if the extension cord is safe to connect a window air conditioner to a wall outlet and an extension cord having a plug which contains circuitry which interrupts the flow of current through the cord if an unsafe leakage current condition should arise. What is also needed is an LCDI that cannot be reset if it is non-functional, that can either indicate an open neutral condition or has the ability to interrupt the circuit during an open neutral condition, and an LCDI that meets these needs with minimal circuitry and power consumption so that it can be fitted into a small form factor.